Tau is a microtubule-binding protein that intrinsically disordered and abundantly expressed in neurons. Certain post-translationally modified forms of tau, such as hyper-phosphorylated and proteolyzed fragments, have been found to accumulate in more than fifteen neurodegenerative disorders, including Alzheimer's disease (AD). These abnormal tau variants are particularly prone to aggregation and their accumulation leads to proteotoxicity, neuron loss and cognitive impairment. Thus, strategies for accelerating the removal of abnormal tau are expected to protect against tauopathies. Cellular protein quality control (PQC) pathways, including molecular chaperones, have been shown to regulate tau homeostasis by controlling its post-translational modification, binding to microtubules and its turnover. However, the molecular details of this process are not yet clear. How do the chaperones distinguish between normal tau and aggregation-prone tau? Which chaperones and co-chaperones are critical for the triage decisions? What molecular events are critical during the earliest stages of tau degradation? The major goal of this project is to understand how the molecular chaperones and other components of PQC make the decision to degrade tau and its variants. Preliminary evidence suggests that the heat shock protein Hsp70 and Hsp90, in combination with some of their associated co-chaperones, detect the appearance of aggregation-prone tau and then make triage decisions to remove these proteins. Based on these studies, we hypothesize that a specific combination of chaperones and co-chaperones bind tau, distinguish between variants and then enact degradation programs. Towards probing that model, we propose two specific aims: (1) perform co-immunoprecipitation and mass spectrometry studies on normal, cycling tau and tau that has been acutely targeted for proteasomal degradation to identify proteins that are specifically associated with the degradation fate and (2) test whether specific Hsp40 co-chaperones selectively recruit Hsp70 to aggregation-prone tau variants. These studies are timely because they make use of new chemical probes that trigger the rapid (~ 10 min) and dramatic (~80%) degradation of tau by the ubiquitin-proteosome pathway. The Gestwicki laboratory has spent the past five years developing, characterizing and optimizing this suite of compounds and, together with the Dickey group, we have shown their activity in cellular and animal models of tauopathy. We propose that, because of their unusually rapid activities, these chemical probes will permit, for the first time, insight into the very earliest stages of the decision to degrade tau. Thus, we expect these aims will provide insight into how chaperone complexes discriminate between tau variants and how protein quality control components shuttle abnormal tau for degradation. The long-term goal of this work is to find new ways of treating AD and other tauopathies by understanding the critical pathways that control tau turnover.